1. Field of the Invention
The present invention relates to a photoswitch of the type electrically switching over the direction in which an optical signal is propagated through a waveguide path.
2. Description of the Prior Art
A waveguide type photoswitch has been proposed in various forms in, for example, JOURNAL OF LIGHTWAVE TECHNOLOGY, LT-4[9], pp. 1324-1327, September 1986-1989(hereinafter referred to as document 1), Nishihara et al "Optical Integrated Circuit", First Edition, Ohm-Sha, pp. 304-307, Feb. 25, 1985, (hereinafter referred to as document 2), and Japanese Patent Application No. 295872/1987 (hereinafter referred to as document 3).
FIG. 1 shows a specific structure of a conventional waveguide type photoswitch adopting a directional coupler system, as disclosed in the document 1. The photoswitch has a substrate 12 exhibiting an electrooptical effect, a pair of waveguide paths 14 and 16 formed on the substrate 12 in close proximity to each other, and electrodes 22 and 24 provided in a coupling region 20 which couples waveguide paths 14 and 16. In the coupling region 20, the refractive indexes n.sub.o of the waveguide paths 14 and 16 are controlled by an operating voltage vo which is applied between the electrodes 22 and 24. While the coupling region 20 has a length l, the electrodes 22 and 24 each has a length le which is smaller than the length l. The length l of the coupling region 20 is selected such that while the operating voltage vo is not applied, light entering the waveguide path 14 or 16 is outputted from the waveguide path 16 or 14, i.e. in a cross condition.
The operating voltage vo applied between the electrodes 22 and 24 destroys the resonance condition of the two waveguide paths 14 and 16. Then, light inputted to the waveguide path 14 or 16 is outputted from the same one 14 or 16, i.e. in a bar condition. Assuming that the length le of the individual electrodes 22 and 24 is equal to the length l of the coupling region 20 (l=le), then the light entering the waveguide path 14 or 16 can be fully outputted from the same waveguide path 14 or 16 if a difference .DELTA.n.sub.o between the refractive indexes of the waveguide paths 14 and 16 satisfies following equation: ##EQU1## where .lambda. is the wavelength of the light in a vacuum.
The refractive index difference .DELTA.n.sub.o mentioned above is proportional to the operating voltage vo, while the electrode capacitance decreases with the decrease of le of the individual electrodes 22 and 24. It follows that the smaller the left member, 2.pi..DELTA.n.sub.o e/.lambda., of the Eq. (1), i.e., the smaller the value of .DELTA.n.sub.o le, the lower the operating voltage vo is. This is successful in reducing the electrode capacitance and thereby enhancing high-speed operation.
When the electrodes 22 and 24 each has a length le which is shorter than the length l of the coupling region 20 (l&gt;le), the waveguide paths 14 and 16 will be brought into a bar condition if the refractive index difference .DELTA.n.sub.o has a particular value expressed as: ##EQU2##
As the value of le/l included in the right member of the Eq. (2) is sequentially decreased toward zero, the result of the Eq. (2) sequentially approaches .pi. and may, therefore, be made smaller than the result of the Eq. (1). More specifically, the product .DELTA.n.sub.o le is smaller in the Eq. (2) than in the Eq. (1).
It has been customary to provide electrodes each having a small length le in the coupling region 20 so as to lower the operating voltage vo and to thereby enhance high-speed operation.
A problem with the prior art photoswitch having the above structure is as follows. Assuming that the refractive index difference .DELTA.n.sub.o is constant, a decrease in the length le of the individual electrodes 22 and 24 results in an increase in the length l of the coupling region 20. Specifically, the refractive index difference .DELTA.n.sub.o and the length l of the coupling region 20 are related as: ##EQU3##
In the above Eq. (3), the value of the left member increases as the value of le/l included in the right member is reduced toward zero. More specifically, assuming that the refractive index difference .DELTA.n.sub.o is constant, the length l of the coupling region 20 increases with the decrease in the length le of the electrodes 22 and 24. This is acceptable so long as the waveguide type photoswitch is used alone. However, when such a photoswitch is integrated in a plurality of stages on a semiconductor substrate, for example, the coupling region 20 having a greater length would undesirably add to the required area of the substrate.
The document 2 teaches a waveguide type photoswitch elaborated to eliminate the above-discussed problem particular to the photoswitch of the document 1. The switch of the document 3 has two waveguide paths arranged side by side on a substrate which exhibits an electrooptical effect. Electrodes are disposed in the coupling region of the two waveguide paths. The distance between the waveguide paths sequentially increases from opposite ends toward the center of the coupling region. In such a switch configuration, the coupling coefficient of the two waveguide paths decreases with the increase in the distance between the waveguide paths. Hence, reducing the coupling coefficient in the position where the two waveguide paths are greatly spaced from each other allows the length of the coupling region to be reduced and the operating voltage to be lowered. Such an approach, however, is not fully satisfactory because it cannot lower the operating voltage beyond a certain level or increase the operating speed to a sufficient degree.